Double debit

I have ADT system im not happy with their services, they are so good at marketing their products and when it comes to serving their clients zero. My monthly premium are debited on the 20th of every month, i woke up this morning i was debit R628.60 of which i was suppose to be debited only R300. When i called in i was told about R328.60 which is a once off. I never signed any documents nor phone call that was informing me about this amount. Bad service and communication!